Although death certificate data are limited by possible misdiagnosis, incomplete case ascertainment, errors in coding, etc., detailed mortality information on some neurologic diseases for the entire U.S. is not available. Analysis of mortality data can be particularly useful for some neurologic diseases because these may contribute to death indirectly. Since there are no uniform criteria for what constitutes the underlying cause of death in patients, it is important to examine all deaths in which a disease is listed as an underlying, immediate, associated, or contributory cause of death to get more complete information about the relationship between the disease and death. Association of diseases occurring at the time of death was also studied for all deaths occurring in the U.S. for many neurologic diseases. Diseases occurring together may provide important information in the search for etiology of diseases. Such detailed analysis of mortality data have been done for Alzheimer's disease and related diagnosis, motor neuron disease, hereditary ataxia, and Down's syndrome, for 1971, 1973-1978. The overall patterns which have emerged have been useful in evaluating trends over time and in formulating etiologic hypotheses.